Gas exchange valves of an internal combustion engine can be operated in a variable manner with different opening and closing times and different valve opening lifts. A valve control system of this type is described in DE 42 30 877 A1. In this case, a cam carrier having two different cam contours is arranged on a camshaft in a rotationally fixed but axially displaceable manner. In accordance with the axial position of the cam carrier, a cam contour is operatively connected to the lift valve via an intermediate member (transfer lever). The axial displacement of the cam carrier for changing the valve parameters takes place during the base-circle phase against the effect of a return spring by means of a thrust collar.
A drawback in this case is the high installation space requirement needed for adjusting the cam carrier. These solutions can therefore only be used in the case of comparatively large cylinder spacings, in order to be able to accommodate the corresponding components. The high inertia forces which occur during the adjustment process and are required for displacing the cam carrier or the adjustment members are a further drawback. The changeover to a corresponding cam contour can only take place in a cylinder-selective manner. A valve-selective changeover is not possible.
DE 100 54 623 A1 describes a device for changing a cam carrier on a camshaft for actuating gas exchange valves, with which device the cam carrier is guided on the camshaft in an axially displaceable manner. In accordance with the position of the cam carrier, the gas exchange valve is operatively connected to different cam contours. The cam carrier is adjusted via an adjustment element in cooperation with a gate track. In this case, the adjustment element is a radially outwardly displaceable pin which when extended cooperates with at least two gate tracks formed in a guide part which is arranged through approximately 180° about the cam carrier.
A drawback of this solution, in addition to the additional installation space for the guide part, is that to change over to another cam contour the pin must be moved out of the camshaft and engaged in an axially displaceable shifting gate. After the shifting operation the pin must be moved in again. This construction is very expensive in terms of parts and production and there is the risk of damage to the camshaft by shifting errors of the pin. A further drawback is that the required adjustment time of the pin limits the engine speed. In addition, the adjustment depends on the oil pressure in each case.
DE 195 20 117 C2 also describes a valve train of an internal combustion engine, in which valve train an axially displaceable cam carrier comprising at least two different cam tracks is arranged on the camshaft in a rotationally fixed manner. The cam carrier is adjusted via an adjustment member which is guided inside the camshaft. The shaft-like adjustment member is displaced inside the camshaft against the pressure of a spring by a dual-action hydraulic or pneumatic piston-cylinder unit arranged at the end of the camshaft. The adjustment member is connected to a driving piece which penetrates an elongate hole arranged axially in the camshaft and discharges into a hole in the cam carrier.
The drawback of this solution is that it is only possible to displace a plurality of cam carriers arranged on the camshaft simultaneously via the axial displacement of the adjustment member. Different shifting of individual cam carriers on the camshaft is not possible. A further drawback is that the spring element is always tensioned in the case of a shift position in which an outer cam is engaged with the gas exchange valves. High lateral frictional forces therefore occur between the driving piece and the guide track arranged on the adjustment member. This results in increased wear and possible shifting errors associated therewith. A further drawback is that the acting spring forces must be set precisely in order to avoid shifting errors, in particular when shifting back to the medium cam profile in the case of three different cam profiles.